The properties of polymers depend upon the properties of the catalyst used in their preparation. In catalysts for gaseous polymerization, control of the shapes, sizes, and the size distribution of the catalyst is important to ensure a good commercial workability. This is particularly important in gas phase and slurry polymerization. For example, in order to produce copolymers of 1000 μm in size, a catalyst particle size of about 30 μm to about 50 μm is generally preferred for use in the polymerization. In the copolymerization of olefins, a catalyst with a well-developed system of pores in its structure is often also advantageous. Additionally, a catalyst should have good mechanical properties to resist wear during the polymerization process and to ensure a good bulk density of the product polymer. One important aspect relating to the development of a polymerization catalyst is, therefore, the provision of a process for production of a catalyst that allows control and adjustment of the structures and sizes of the catalyst particles and particle size distribution while remaining a fairly simple process.
Spray-drying is one technique for the simple assembly and shaping of polymerization catalysts. In spray-drying, liquid droplets containing dissolved and/or suspended materials are ejected from a flywheel or a nozzle. The solvent evaporates leaving behind a solid residue. The resulting particle size and shape is related to the characteristics of the droplets formed in the spraying process. Structural reorganization of the particle can be influenced by changes in volume and size of the droplets. Depending on conditions of the spray drying process, either large, small, or aggregated particles may be obtained. The conditions may also produce particles that are compositionally uniform or mixtures of solution components. The use of inert fillers in spray-drying can help control shape and composition of the particles.
Numerous spray-dried olefin polymerization catalysts containing magnesium and titanium and production processes utilizing them have been reported. Inert fillers are sometimes used to control the shape of the resulting catalyst precursor or catalyst support by providing bulk to the solid composition. However, fillers that are small compared to the dimension of the sprayed droplet or dried particle are used because relatively large fillers lead to the formation undesirable amounts of very small catalyst particles. Such small catalyst particles contaminate the resin with particulate residues that lead to gel formation. While the small fillers reduce problems with gel formation, when they are formed into larger catalyst particles, diffusion related problems arise during subsequent activation processes and during the polymerization process. For instance, during the activation process, the cocatalyst tends to activate only a shallow region of the particle near the particle surface because the cocatalyst is unable to penetrate the interior regions of the particles. These relatively large and dense particles also inhibit the diffusion of monomers into the interior region catalyst particles during the polymerization process.